To protect people and valuables in buildings or rooms against forced entry, lockable doors are used. Various types of locks may be used to secure a door in the closed position. Typically, a lock secures the free edges of the door to the door frame using one or more deadbolts. Such a lock provides no support for the door itself, and the central portion of the door may be broken if subjected to blows. Such a lock also must be completely disengaged in order partially to open the door, and once the door is partially opened (for example, when an occupant wishes to speak to someone at the door or to receive a package), it is relatively easy for an intruder to force the door open further to gain entry.
To brace the door and to secure the door in the closed position, a cross brace may be placed horizontally across the door and secured on either side of the door to the door frame or wall.
Alternatively, an angle brace may be used to support the door and to secure it in the closed position. One end of the angle brace is attached to the door at a distance from the free vertical edge of the door, typically near the door handle. The other end of the angle brace is attached to the floor at a distance from the door such that the angle brace lies at an angle sufficient to brace the door shut. An angle brace may be constructed so as to be adjustable in length by using a tube or bar that is slidable within a second tube of slightly larger diameter. A series of holes are drilled through both, and a pin or bolt is placed through matching holes to fix the length of the angle brace before attaching it to the door or floor.
However, like conventional locks, cross braces and angle braces must be disengaged prior to partial opening of the door.
To secure the door in a partially open position, a door chain may be used. However, a door chain is generally not strong enough to withstand a forceful blow. If the chain used is strong enough, it is usually impossible to attach the chain strongly enough to the door and door frame to withstand a forceful blow to the door. Another limitation of a door chain is that it provides no support to the door, and it can be severed once the door is partially opened.
A door stop may also be used to prevent a door from moving. While it is possible to build a door stop that will withstand forceful blows to the door, the door itself is placed in a weak position since it is supported only by its hinges and by the door stop under the lower edge of the door.